1. Technical Field
The present invention relates to an atomic oscillator.
2. Related Art
An atomic oscillator based on an EIT (Electromagnetically Induced Transparency) system (also called a CPT (Coherent Population Trapping) system) is an oscillator using a phenomenon in which when two kinds of resonant lights having coherency and having specific wavelengths (frequencies) different from each other are simultaneously irradiated to an alkali metal atom, absorption of the resonant lights is stopped.
As shown in FIG. 14, it is known that the interaction mechanism between the alkali metal atom and the two kinds of resonant lights can be explained in a Λ-type three-level model. The alkali metal atom has two ground levels, and when resonant light 1 having a frequency corresponding to an energy difference between the ground level 1 and the excited level or resonant light 2 having a frequency corresponding to an energy difference between the ground level 2 and the excited level is individually irradiated to the alkali metal atom, light absorption occurs as is well known. However, when the resonant light 1 and the resonant light 2 are simultaneously irradiated to the alkali metal atom, a superimposed state of the two ground levels, that is, a quantum interference state occurs, the excitation to the excited level is stopped, and the transparency phenomenon (EIT phenomenon) occurs in which the resonant light 1 and the resonant light 2 pass through the alkali metal atom. For example, in a cesium atom, a ground state of a D2 line (wavelength is 852.1 nm) is split into two states having levels of F=3 and F=4 by an hyper fine structure, and the frequency corresponding to the energy difference between the ground level 1 of F=3 and the ground level 2 of F=4 is 9.192631770 GHz. Accordingly, when two kinds of laser lights having wavelengths of about 852.1 nm and a frequency difference of 9.192631770 GHz are simultaneously irradiated to the cesium atom, the two kinds of laser lights become a resonant light pair and the EIT phenomenon occurs.
When the two kinds of lights different in frequency are irradiated to the alkali metal atom, the light absorption behavior is abruptly changed according to whether or not the two kinds of lights become the resonant light pair and the alkali metal atom produces the EIT phenomenon. A signal representing the abruptly changing light absorption behavior is called an EIT signal, and when the frequency difference between the resonant light pair accurately coincides with the frequency (for example, 9.192631770 GHz for the cesium atom) corresponding to the energy difference ΔE12 between the two ground levels, the level of the EIT signal indicates the peak value. Then, an oscillator with high accuracy can be realized by detecting the peak value of the EIT signal and by performing frequency control so that the two kinds of lights irradiated to the alkali metal atom become the resonant light pair, that is, the frequency difference between the two kinds of lights accurately coincides with the frequency corresponding to ΔE12.
It is known that when a magnetic field is applied to the alkali metal atom, each of the two ground levels is split (call Zeeman splitting) into plural levels according to the magnetic quantum number. That is, since the energy difference ΔE12 between the two ground levels changes according to each magnetic quantum number, when the frequency difference between the resonant light pair is changed, plural EIT signals appear. At this time, when the intensity of the magnetic field is very low, the plural EIT signals are superimposed on each other and one EIT signal having a wide line width is obtained. Thus, it becomes difficult to accurately detect the peak value, and the frequency accuracy is deteriorated. However, it is difficult to completely eliminate the influence of magnetic field due to disturbance. Then, attention is paid to the fact that in a low magnetic field, even if the intensity of the magnetic field is changed, the energy difference ΔE12 between the two ground levels for the magnetic quantum number 0 can be regarded as being almost constant, an atomic oscillator is proposed in which the frequency accuracy is improved by applying a low magnetic field to an alkali metal atom to such a degree that plural EIT signals are completely separated and by detecting the peak value of the EIT signal for the magnetic quantum number 0.
U.S. Pat. No. 6,265,945 is an example of related art.
However, it is very difficult to cancel the influence of the disturbance and to stably control the weak magnetic field. Further, even in the weak magnetic field, since the energy difference ΔE12 between the two ground levels changes subtly, it is difficult to further improve the frequency accuracy in the related art system.